After a hydrocarbon (oil and/or gas) well is drilled, a steel casing is run quickly into the wellbore. The casing has a smaller diameter than the wellbore and is landed as quickly as possible (for reasons of cost and hole stability, amongst others). After the casing has been installed, cement is normally pumped into the annular space between the casing and the surrounding formation (the “annulus”) to seal it off and ensure that hydrocarbons to not come to the surface via the annulus. The annulus could be cemented over a relatively short (5-10 m) length of casing in order to achieve a leak off test (“LOT”), the “green light” to continue drilling. In addition, a casing or liner hanger packer is installed as a further precaution. The drilling of the overburden (the rock above the oil-bearing region) will continue like this with ever smaller casing dimensions. The length of each section is, amongst other things, a function of the rock properties.
After drilling and casing installation is finished in the overburden and the reservoir section (well construction), the well is completed with tubing before being set on production or injection. It will remain productive until it becomes uneconomic. At this point the well must be decommissioned in a way which minimizes the risk of leakage of hydrocarbons into the environment on a permanent basis. The plug and abandon (P&A) process is often described as re-establishing the cap rock (the overburden) in a manner which will ensure it can withstand reservoir pressure, again, on a permanent basis. In order to do this an effective long term barrier must be proved to exist already, or must be installed in the annulus as well as inside the casing itself. If the section in question was cemented during the well construction (proven by original reports or logging) this may be combined with an inner plug.
If the existing cement is insufficient, then the formation/annulus must be accessed in some way in order to inject cement (or another plugging material) into it, e.g. by perforating the casing using explosive or puncturing it by some mechanical means. Alternatively, the casing may be milled away entirely over some of its length to expose the formation and then a cement plug created spanning the entire wellbore. Both the outlined operations are expensive and time-consuming and both require a high capacity surface package, normally a drilling unit.
In some wells, it is believed that the formation rock in the overburden creeps after the casing is installed, possibly forming an effective natural seal between the overburden formation and the casing. However, in many wells this does not occur. The reasons for this formation creep phenomenon happening (or not happening) are not well understood.